Operational amplifiers are prone to instability and, thus, require some means of frequency compensation to ensure reliable stable operation. FIG. 1 shows a commonly used compensation technique used in an operational amplifier 100. The amplifier 100 includes a transconductance stage 102, a compensation capacitor (CCOMP) 104 and high-impedance resistor (RHI) 106 connected to the gain node n105, and a buffer 108 connecting the gain node to the amplifier output.
The compensation technique used in FIG. 1 includes placing the compensation capacitor 104 at the high-impedance gain node n105 to reduce the amplifier gain at high frequency, thus introduces a dominant pole into the open-loop frequency response of the operational amplifier 100. The dominant pole then occurs at a frequency fp calculated as follows:
 fp=1/(2πRHICCOMP)
The minimum value of compensation capacitor (CCOMP) required to guarantee stable operation depends on many factors, including external feedback and the load impedance at the amplifier's output.
Often two of the operational amplifiers 100, as shown in FIG. 1, will be interconnected to form a differential driver circuit, which contains both a differential and a common-mode signal path. An example of two such interconnected amplifiers 200 and 202 is shown in FIG. 2. In FIG. 2, a common mode signal is applied by voltage source (VCM) 220 to the non-inverting inputs of amplifiers AMPA 200 and AMPB 202, while a differential mode signal is supplied by voltage sources 232 and 230. Any arbitrary voltage input to the circuit in FIG. 2 can be represented as a sum of common-mode and differential input voltages. Feedback resistors 204 and 206, each having a resistance value RF, are connected between the output of amplifiers 200 and 202 and their inverting inputs. A gain resistor 208, having a resistance 2RG, is further connected between the inverting inputs. A load impedance having the resistance RL is connected between the outputs OUTA and OUTB of the respective amplifiers AMPA 200 and AMPB 202.
To illustrate why the compensation scheme of FIG. 1 is not optimal for the circuit of FIG. 2, consider the effect of the two independent signal paths on stability. Common-mode signals will not cause any current to flow through the gain resistor 208 and load resistor 210, and therefore the common-mode and differential signal paths will have different voltage/current feedback levels and load impedances. The differential output voltage VOD and the common-mode output voltage VOC are given as follows:VOD=VDIFF(1+RF/RG)VOC=VCMTherefore the common-mode signal path has a gain of unity and its output is unloaded, while the differential signal path has a higher gain and sees a resistive load.
Because of these differences the two signal paths will have different minimum values of CCOMP required to ensure stable operation. To guarantee stable operation of the entire circuit, the larger of these two values must be used. If, for example, the common-mode signal path requires a higher value of CCOMP, then the differential signal path will be “over-compensated”, thus lowering signal bandwidths and slew rates and limiting overall amplifier performance.
One circuit modification to partially avoid this problem would be to replace the gain resistor 208 of FIG. 2 with the resistors 300 and 302 of FIG. 3 which connect the inverting inputs of amplifiers 200 and 202 to ground. With such a circuit both differential and common-mode signals cause current to flow through RG. Therefore both signal paths see the same feedback levels and have the same closed-loop gain, and will require roughly the same value of CCOMP. However, the load impedance is still different for the two signal paths and some over-compensation is unavoidable. Additionally, the connection to ground in the circuit of FIG. 3 has several undesirable properties. Common-mode offset and noise voltages will be fully amplified, and because the two amplifiers are now isolated their output voltages and currents will not necessarily track each other. These effects will compromise differential signal performance.